Rechargeable battery packs are used with many different types of portable electrical and electronic devices. Examples of such devices include hand-held communications devices, such as cellular telephones, and power tools, such as cordless drills. In designing rechargeable battery packs for such devices there are usually two conflicting goals: maximizing the energy storage capacity and minimizing the weight of the battery pack. For many years a typical rechargeable battery pack comprised nickel cadmium battery cells. More recently another type of battery cell using an electro-chemical system commonly referred to as nickel metal-hydride has gained acceptance because of its increased energy capacity to weight ratio compared with nickel cadmium type battery cells. In the past couple years, a third type of battery cell, whose electro-chemical system is commonly referred to as lithium ion, has gained notoriety because it offers an energy capacity to weight ratio that is significantly superior to both nickel metal-hydride and nickel cadmium type battery cells.
However, the certain precautions must be taken when designing a battery pack based on lithium ion cells. For one, over-discharging a lithium ion battery cell can irreversibly damage the cell and significantly shorten its cycle life. Thus, nearly all manufacturers of lithium ion battery packs have incorporated a circuit in the battery pack to prevent over-discharge, typically by preventing the battery pack from sourcing electrical current once the battery voltage of the battery cell falls to an under-voltage level. This is usually done by providing an undervoltage switch in series with the battery cell(s) that opens once the battery voltage reaches the undervoltage level. Likewise, charging a lithium ion battery cell above an over-voltage level, typically about 4.2 volts DC, can shorten the cycle life of the cell. Furthermore, charging a lithium ion battery cell beyond the over-voltage level, in extreme circumstances, can result in a safety hazard as the cell may experience a thermally induced rupture. To prevent over-charging of lithium ion cells, two measures are typically implemented. First, the associated battery chargers are designed such that their output voltage is limited to prevent the battery voltage from exceeding an over-voltage level. This type of limiting can be achieved either through hardware design, software design, or both. Second, an over-voltage switch and associated control circuit are provided in the battery pack. If the battery voltage reaches a level beyond a level that an appropriately designed charger would limit, the control circuit causes the over-voltage switch to open, thus preventing any further charging of the battery cell. Both under-voltage and over-voltage circuits are well practiced in the art.
However, it is conceivable that both the battery charger and the over-voltage circuit of the battery pack may fail, resulting in the lithium ion cells being unprotected from an overcharge condition. In the unlikely event that a double failure such as this occurs, a safety hazard can exist. Therefore there is a need for a means to prevent the occurrence of an unsafe condition, even if a failure occurs in both the charger and the over-voltage circuit of the lithium ion battery pack.